Marine algae from the genus Caulerpa
get most of the attention in the reef aquarium hobby. Caulerpa
species are fast-growing, decorative algae that have been
mainstay livestock items in the hobby for years. They are
an integral component of some current methods of natural filtration
such as the mud refugium systems. While it may be hard to
argue against Caulerpa's effectiveness as a nutrient
exporter, these algae are prone to grow out of control in
aquaria and ultimately cause as many headaches as they relieve.
A viable alternative for the reefkeeper interested in a group
of decorative algae with growth rates matching Caulerpa
are the "cactus algae." The common name "Cactus
algae" is ascribed to many members of the genus Halimeda.

Halimeda are green algae from the
phylum Chlorophyta and are classified in the order Bryopsidales
along with Caulerpa. Bryopsidales are somewhat unique
in that they are coenocytes; the entire organism lacks individual
cells. While a normal plant cell is tiny, enclosed in a cell
wall, and contains one nucleus with the genetic material,
coenocytic algae can be thought of as a single giant cell
with multiple nuclei. Halimeda belong in either the
family Halimedaceae or Udoteaceae, along with other close
relatives including Udotea and Penicillus (commonly
know as shaving brush algae). This family's distinguishing
characteristic is the ability of its members to synthesize
calcium carbonate from seawater, much like stony corals and
the beautiful purple coralline algae do. Roughly thirty modern
species of Halimeda are known, and several others are
known only from the fossil record.

A typical Halimeda alga is a flexible
string of flattened leaf-like structures often referred to
as segments. Each segment is a deposit of calcium carbonate
covered by the algal protoplasm and connected to its neighbors
by a thin strand, giving the plant its flexibility. Some species,
such as H. copiosa, take on the form of long necklaces,
as if green disk-shaped charms had been fitted on a thin chain.
The most common species, H. tuna and H. discoidea,
have larger segments and grow in somewhat shorter chains.
Another species common to aquaria is H. opuntia. It
has very small segments and tends to form dense, shrub-like
masses. Halimeda opuntia is named for the Opuntia
genus of cactus, which includes Halimeda's terrestrial
look-alike, the prickly pear.

Halimeda incrassata. Photo by Kirby Adams.

Photo by Kirby Adams.

Halimeda inhabit a range of marine
habitats from sandy bottom areas to rocky reef structures,
and have been reported from depths up to 150 meters (500 feet).
The ability to thrive at depths where light radiation is minimal
suggests an evolved trait that occupies a niche with few other
photosynthetic organisms. Since algae are not vascular plants
and do not have true roots, they will grow wherever they settle
in an aquarium. A more natural appearance and growth form,
however, can be maintained by placing sand-dwelling species
on the substrate and reef dwellers on the rocks. The substrate
'rooted' species have holdfasts that can extend 10cm (4in)
or deeper into a sandy bottom while the others attach themselves
to rocks with a series of short holdfasts similar to those
of the commonly kept Caulerpa species. Older aquarium
hobby books suggested Halimeda were quite difficult
to maintain in a closed system, but today most of the available
species are considered very hardy. Lighting is not extremely
critical to these algae and any illumination sufficient for
a reef aquarium will be more than enough to meet their requirements.

The most stunning characteristic of Halimeda
is their rapid growth rate and the accompanying deposits of
calcium carbonate. These algae have been found to be the primary
reef-building organism of the tropical seas, erasing the notion
that stony corals are the champion reef engineers. On the
Great Barrier Reef a 2,000 km2
(1,250 sq. mile) area covered with coarse gravel from 10-15m
(33-50 ft) deep was found to be primarily Halimeda
fragments with vast areas comprising as much as 98% algal
deposits (Drew 1986). The same study on the Great Barrier
Reef showed that huge meadows of Halimeda produced
up to 2 kg calcium carbonate per m2
every year. That means that a patch of Halimeda the
size of an average living room would produce more than 100
pounds of aragonite in a single year. Halimeda accomplish
this feat with rapid growth, doubling the biomass of a colony
every 15 days (Drew 1983). Considering that 90% of the plant
is nothing but aragonite, it's easy to see how it builds up
so quickly.

Photo by Kirby Adams.

The rapid growth and calcification rates
of Halimeda add to their appeal and usefulness in the
marine aquarium. The growth indicates these algae could be
every bit as effective as an instrument of nutrient export
as the more popular Caulerpa. Regular harvesting of
any algae will export nitrogen, phosphorus, and other organic
compounds, and faster growth rates lead to more frequent harvests.
The calcification abilities of Halimeda make them useful
as a barometer of calcium and alkalinity parameters in a closed
system, but can also render them somewhat dangerous in a system
where the calcium demand is already high. Rapidly growing
masses of calcareous algae are good indicators that the amounts
of available calcium and carbonate in the water are adequate
for calcification, a critical consideration for the maintenance
of stony corals. Healthy Halimeda that is properly
calcifying will have segments that are somewhat rubbery but
not spongy. This indicates production and deposit of aragonite
crystals and is as reliable as any calcium and alkalinity
test kits you can buy at the corner store. The calcification
does, however, remove the valuable calcium and carbonate building
blocks from the system and can therefore greatly increase
the demand for these elements. Halimeda must essentially
be treated as a stony coral regarding their demand for proper
levels of calcium and carbonate hardness. This isn't much
of a concern for the average reef-keeper who already maintains
these levels religiously, but it must be kept in mind that
large masses of Halimeda can drain a system of both
calcium and carbonate hardness rapidly if these materials
aren't added regularly.

Photo by Kirby Adams.

The presence of gigantic unmolested meadows
of Halimeda in the Great Barrier Reef indicates that
they are distasteful to potential grazers, or perhaps repel
herbivores in some fashion. Both of those presumptions have
been proven true. The plants' calcareous nature makes them
a less appetizing meal to grazing fish such as surgeonfishes
than more succulent algae. Halimeda go a step further
to ward off aragonite-munching herbivores, such as parrotfish,
by synthesizing noxious and potentially toxic secondary metabolites.
The aptly named halimedatrial and halimedatetraacetate are
diterpenoid compounds that appear to give Halimeda
an extremely noxious taste and could prove toxic in large
quantities (Paul and vanAlstyne 1988). Some of the few predators
that threaten Halimeda are the chloroplast-thieving
sacoglossan slugs. Lettuce slugs steal chloroplasts from algae,
killing or damaging the algae and rendering themselves photosynthetic.
Halimeda are vulnerable to this robbery, but have developed,
perhaps inadvertently, a defense against even this type of
grazing. The chloroplasts of the green tissue, which are normally
clustered near the surface of the thallus, migrate more deeply
into the tissue at night, leaving little for a marauding slug
to pilfer (Drew 1990).

Caulerpa fanciers are usually familiar
with those algae's method of sexual reproduction. A colony
will expel its gametes along with all of its cytoplasm, leaving
a snow white or transparent (and very dead) clump of algae
and a pea soup green aquarium in its wake. Halimeda's
sexual reproduction is similar, but with the added benefit
of a known warning indicator. Hours before releasing gametes,
the algae will turn pale white with dots of very dark green
or almost black along the edges of the thalli. The dots are
called gametangia and contain all of the contents of the living
plant, concentrated in tiny capsules. This creation of the
gametangia is called sporulation. Shortly thereafter, the
gametes are released in a fashion similar to Caulerpa's.
Plants that reproduce in this fashion, with the entire plant
becoming reproductive, are said to be holocarpic. These sexual
events have been blamed for sudden deaths of tank inhabitants,
and the secondary metabolites of the algae are often fingered
as the cause. While this is certainly possible, it seems more
likely that fish and invertebrates succumb to oxygen deprivation
during these gamete-releasing events. The entire content of
large masses of algae is concentrated in millions of short-lived
gametes, putting an incredible oxygen demand on the system.
Under these circumstances, immediate partial water exchanges
combined with increased aeration and protein skimming are
called for. If an ORP meter is used, the severity of the situation
can be assessed more easily, and oxidizers such as ozone and
potassium permanganate can be used. This is a worst-case scenario,
however, and most sexual reproduction occurs without the destruction
of the entire system, or any ill effects at all. It has been
suggested that a lack of pruning and a deficiency of iron
lead to sporulation (Tullock 1997). Personal observations
indicate regular pruning to be a deterrent to sporulation,
but the lack of an essential nutrient such as iron triggering
sexual reproduction seems questionable. After sporulation,
the remaining Halimeda can be left in the aquarium,
as they are essentially nothing but aragonite and can become
part of the natural substrate. If you choose to remove a dead
clump of Halimeda, be certain that it is actually dead.
The chloroplast migration mentioned above leaves the plant
noticeably pale at night and can mimic the look of a spent
plant following gamete release. It is also important to note
that white tips on the terminal segments indicate new growth
and are not harbingers of death or reproduction.

Halimeda opuntia. Photo by Kirby Adams.

Halimeda's resistance to grazing
and their tendency not to overrun and damage sessile invertebrates
make them a far better choice as a decorative and functional
alga than Caulerpa. Factor in their nutrient export
capabilities and service as an alkalinity test and you have
the perfect marine plant for the home aquarium!

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